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Internal cracking, damage and defects can develop in a material over time and resul-t in failure of the structure. Although there are several non-destructive evaluatio-n techniques for detecting damage in structures, to date such techniques have bee-n difficult and expensive to implement during the service life of a structure. Comp-osite materials, such as those used in planes, bridges and buildings, provide oppor-tunities for overcoming these problems. Composites provide room for engineering ne-w functionality into a material, such as an inner "nervous system" that could rela-y damage information.


Recently, engineers at Massachusetts Institute of Technology (MIT), US, embedded su-ch a sensing network made of carbon nanotubes (CNTs) into a traditional fibre-reinf-orced laminate architecture, by placing aligned CNTs throughout the architecture to create a hierarchical nanoengineered material.

When electrical current flows through the CNT network, the temperature of the mater-ial increases from Joule heating. If damage that disrupts this conductive network e-xists somewhere in the material, a localized change in electrical current result-s,   which, in turn, manifests as a localized change in temperature.

A simple low-cost thermal camera or even night-vision goggles enable real-time visu-alization of damage in the composite. Reporting their results in the journal Nanote-chnology, the researchers show that several kinds of damage can be detected in nano-engineered composites using this non-destructive in situ inspection technique inclu-ding barely visible damage, internal damage produced by impact and other forms of   otherwise hidden damage.

The structure is the sensor

This method has numerous advantages over existing non-destructive evaluation techni-ques. First, the structure itself is the sensor – no wiring, sensor grids or extern-al heaters are necessary. In addition, this sensing technique is truly multifunctio-nal – the nanoengineered composites are stronger and tougher mechanically, with th-e sensing benefit coming as a secondary effect. Finally, high-resolution inspection is easily obtained and, in the case of nanoengineered composites, with only a tiny a-mount of power.

The team believes that the strategy employed in the study can therefore serve as th-e basis for new, improved inspection techniques for monitoring future generations   of safer vehicles and infrastructure.